Tube length and altitude

[K C]

Has anyone done any testing in relation to tube lengths and the altitude of the shell burst when firing reloadables?

I have some HDPE tubes I was planning on using in some racks, but they are only 12" tubes. The plug in the bottom of these is 1.5", and this leaves only about 10.5" of actual tube length.

When you factor in the fact that an Excalibur cannister (I have a case of Excals I'm planning on shooting this 4th) has a length of almost 4 inches, it just doesn't seem to leave much tube length for the shell to gain velocity.

Does any one here who shoots reloadables (and especially Excaliburs) ever use tubes longer than 15", and does it make a difference?

I was wondering if making my own HDPE tubes at, say, 17" or 18" would give the shells more altitude.

[BigDave]

This is controversial, some say standard tubes are good for all some see a difference.

Dewey says 12" tubes are always fine for all shells.

I've noticed that big shells like Excals are more likey to have a willow star touch down in a 12" tube.

A excal is 3 times the weight of a standard ball shell ~140 grams versus ~45 grams.
It has more lift powder to provide lift, but that extra lift powder will require more space for that extra powder to burn fully.

Another way to look at it is what is the Excal designed to be fired in.

A 12" tube has 10.5" interior
A 15" tube has 13.5" interior
A excal kit tube has 12.75" interior.

The 15" tube is much closer.

[vinito]

I just brought my stash home this seekend and I got to talking on chat about what came with what & measuring stuff.

The Excalibur kits I got came with a one-piece molded tube & base. The tube length is only about 12-1/2" (no plug to consider since it's just a sealed end on the bottom). I've never had excals before, but if they used to come with 15" tubes then they've changed for this year.

I tested a few and, besides being awesome, they had plenty of height for me. Still seems like the impressive lift charge they have might benefit from some more tube length, but the breaks on the ones I shot came nowhere near to touching the ground. It might make more difference if viewing from a pretty good distance.

BTW, inside diameter of the supplied excal tubes measures about 1.93"

[Dragon]

The Excalibur kits I got came with a one-piece molded tube & base. The tube length is only about 12-1/2" (no plug to consider since it's just a sealed end on the bottom). I've never had excals before, but if they used to come with 15" tubes then they've changed for this year.


I was just cutting the bases off of my case of Excal to use in my extra racks. I tried someones idea about putting the base on a bench and whacking it with a hammer. I think that is for the SnM tubes only. It did not work in the least. I ended up getting the scroll saw out and tilting the table at about 10 degrees to get the base off easiest. Anyhow I was wondering how big the plug was, so I measured it.

Outter measurement was 13 1/8
Inner measurement was 12 7/8

I was really shocked that the plug was only 1/4" thick8o . Since we have to use a 1.5" wooden plug.

[vinito]

The way mine are made (haven't seen any others) is that the bottom isn't plugged. It's an integral molding such that the tube and bottom are all one piece. Since the tube wall is only about 1/4" and the mold is likely to be slightly filleted in the corners at the bottom, a 1/4" bottom is probably fine. Just a wild-ass-assumption, but I feel secure using the tubes.

[Dragon]

Ok, I mis-used the word plug. It is the molded bottom of the tube. It is HDPE bottom not a wooden plug bottom. Sorry for any confusion.:confused:

[Fire-Man]

It depends, would you rather have higher breaks, which makes the break appear smaller, or would you rather have lower breaks (still safe though) which makes the breaks appear bigger?:confused:

[Kevin13]

It depends, would you rather have higher breaks, which makes the break appear smaller, or would you rather have lower breaks (still safe though) which makes the breaks appear bigger?:confused:


Varying the length of tube will probably never give you this scenerio while keeping a safe height. For example, the height a shell obtains between a 12" and 15" length mortar is minimum at best. A test was done between a 15" and 26" length mortar and there was no noticable difference at all.

[UnkleSAM]

There is such a thing as too long a mortar too. Essentially what we are dealing with is a pneumatic pump. If you consider a scenario where the shell was perfectly fitted (there was no blow-by) to a mortar of sufficient length the following would happen. The lift charge would propell the shell (piston) to a distance where the pressure behind the shell equalled the outside atmospheric pressure. From this point on, the shell's inertia would cause it to continue down the mortar, but it would be decelerated as the back pressure became less than the front pressure until it stopped, and indeed reversed direction. The actual math is more complicated than this due to friction, acceleration, mass, gravity and front pressure caused by the accelerating shell, but you get the idea.

The long and the short of it is that 1 gram of black powder will create about .274 liters of gas - almost exactly equal to the volume of a plugged 2" x 15" SDR-11 mortar. Factor in blow-by, friction, mass, etc. and you'll see that to optimize a shell's lift you need to very precisely match the tube length (and more importantly diameter) to the physical characteristics of the shell, and its lift charge. Considering how little lift charge a consumer shell has it's easy to see that too long a tube can actually cause decrease in performance. Tube length (above minimum limits) is no more important then several other factors in determining the exit velocity of a shell.

[mindless2]

Sometime next weekend I can do a side by side test of shells launched out of 12" and 15" tubes. I will get a video of it, to see if there will be any difference.

[BigDave]

Sometime next weekend I can do a side by side test of shells launched out of 12" and 15" tubes. I will get a video of it, to see if there will be any difference.

I don't expect there to be any except in Excals, and large multi break shells.
Maybe in Kylin King Super Mags or similar.

In other words, only very heavy tall shells.

[RudyG]

I saw very little or no difference at all in the Excal's. So I ordered all my racks with 12' HDPE tubes. I will test the Kylin King Super Mags this weekend. I expect no difference again, but it's always fun to do a little testing.;)